The present invention relates to a method for providing a source of penetrating radiation for a scanning imaging system having a varying collimation path, wherein the target of an x-ray tube is scanned in such a manner as to reduce the focal spot size while maintaining stability of the origin of the penetrating radiation with respect to the collimation path.
Various x-ray systems, such as those used for backscatter imaging of articles or containers, employ a rotating collimator wheel to create a scanning beam of x-rays, such as shown, for example, in U.S. Pat. No. 5,764,683, which is herein incorporated by reference. One method for generating a scanning beam of x-rays is shown in FIG. 1. An x-ray tube 60 is in the center of a mechanical scanner 66 which includes a rotating wheel 61 of material substantially opaque to x-rays except in the paths defined by hollow spokes 62. X-ray tube 60 generates x-rays by bombarding a target with a beam of particles, typically electrons, emerging from a direction having a component transverse to the plane of the drawing. X-ray beams 64 sweep in the plane of the drawing over an object 68, which may be a container in which articles are concealed, for example. Scattered x-rays 72 are detected by scatter detector 74. An image of the intensity of x-rays scattered by object 68, or transmitted through object 68, may be obtained as object 68 is conveyed through the scanned x-rays by a conveyor 70. Alternatively, object 68 may be fixed while x-ray beams 64 and/or detector 74 are swept.
Mechanical scanner 66 is typically a massive and ponderous component since rotating wheel 61 must be opaque to the penetrating radiation being scanned and is thus typically made out of lead of sufficient thickness to stop the most energetic x-rays produced by x-ray tube 60. It is thus advantageous that the collimator wheel be small. Reduction of the size of the collimator wheel may be limited by the size of the region of the x-ray tube target onto which the electron beam is focused. The size of the emission region may be limited in turn, for a specified output flux, to a minimal size governed by the power dissipation capacity of the x-ray tube target.
For x-ray imaging systems that use a beam of x-rays, the size of the focal spot on the x-ray tube target is a limiting factor for the resolution of the image that is obtained. A smaller focal spot results in a higher image resolution. The size of the focal spot is limited, however, by the amount of power that can be dissipated in the target by the incident electron beam. If the focal spot is large, the electron energy is dissipated over a relatively large area of the target, without overheating any one area of the target. If the focal spot is too small, the energy density in the focal spot is very high which can lead to overheating and melting of the target in a relatively short time. A small focal spot can be safely used, however, if the focal spot is not stationary on the target, but is raster-scanned across the target. This prevents the focal spot from being incident on any one area of the target for a period exceeding the dissipation capacity of the material of the target and allows the focal spot to be moved to another area of the target before any damage occurs.
In accordance with a preferred embodiment of the present invention, there is provided a method for producing a scanned beam of penetrating radiation. The method has the steps of:
a. illuminating a portion of a target with a beam of particles, the target having an effective center, the portion of the target illuminated by the beam of particles comprising a focal spot having a centroid, the illumination being such as to create a beam of penetrating radiation;
b. directing the beam of penetrating radiation through a collimating path having an instantaneous direction defining a line; and
c. scanning the beam of particles across the target in such a manner that the centroid of the focal spot lies on the line defined by the instantaneous direction of the collimating path.
In accordance with other embodiments of the present invention, the beam of penetrating radiation may be a pencil beam, the penetrating radiation may include x-rays, and the beam of particles may be an electron beam. The step of directing the beam of penetrating radiation may include steering the particle beam across the target by electromagnetic means.
In accordance with yet other embodiments of the invention, there is provided an apparatus for scanning a beam of penetrating radiation across an object. The apparatus has a source of penetrating photons having a beam of particles incident upon a target at a region of incidence such that penetrating photons are emitted from the region of incidence into a penetrating beam. Additionally, the apparatus has a collimator that allows propagation of a beam of penetrating photons along an instantaneous direction defining a line as well as a particle beam steering arrangement for selecting the region of incidence of the beam of particles upon the target in such a manner that the centroid of the region of incidence of the beam of particles lies on the line defined by the instantaneous direction of the penetrating photons. The apparatus may also have a penetrating beam director for defining the instantaneous direction of the penetrating photons. The penetrating beam director may be a chopper wheel, or a slit rotating with respect to a fixed axis, or a pair of slits, the slits counterrotating with respect to a fixed axis.